Refrigeration & Air Conditioning - Mechanical Engineering MCQ

The exit condition after evaporation is saturated vapour which is compressed in the compressor and become superheated (having the maximum temperature) and then passed to the condenser.

When relative humidity of the air is 100%, i.e the air is saturated, the dew point temperature (DPT) equals the wet bulb temperature (WBT), which is also equal to the dry bulb temperature.

So DBT = WBT = DPT

For unsaturated air:

DBT > WBT > DPT

After Condenser condenses the refrigerant, it is passed through an expansion valve to reduce its pressure to evaporator pressure. But while doing so, some of the liquid converts to vapor again. So, to avoid vapor to enter evaporator, a flash chamber is used.

A flash chamber is a device which separates liquid from vapors. Only liquid is then passed to evaporator and the vapors will be passed to the compressor directly.

To ensure perfect pressure reduction and maximum liquid to be passed through evaporator, multiple no of flash chamber can be used in refrigeration cycle.

Flash chamber reduces the size of evaporator and it does not have any effect over COP and the system.

Humidity ratio/Specific humidity, w (kg/kg) of a given moist air sample is defined as the ratio of the mass of water vapour (m_w) to the mass of dry air (m_a) contained in the sample.

Enthalpy of moist air = Enthalpy of 1 kg of dry air + Enthalpy of water vapour associated with 1 kg of dry air

h = h_a + wh_v

Electrolux refrigerator is a domestic refrigerator and is the best known absorption type of refrigerator.

• The ammonia liquid leaving the condenser enters the evaporator and evaporates into the hydrogen at the low temperature corresponding to its low partial pressure.
• The mixture of ammonia and hydrogen passes to the absorber into which is also admitted water from the separator.
• The water absorbs the ammonia and the hydrogen returns to the evaporator. In the absorber the ammonia therefore passes from the ammonia circuit into water circuit as ammonia in water solution.
• This strong solution passes to the generator where it is heated, and the vapor given off rises to the separator.
• The water with the vapor is separated out and a weak solution of ammonia is passed back to the absorber, thus completing the water circuit.
• The ammonia vapor rises from the separator to the condenser where it is condensed and then returned to the evaporator.

Humidity ratio/Specific humidity, w (kg/kg) of a given moist air sample is defined as the ratio of the mass of water vapour (m_w) to the mass of dry air (m_a) contained in the sample.

Relative humidity (ϕ):

It is defined as the ratio of partial pressure of water vapour (p_v)in a mixture to the saturation pressure (p_s) of pure water at the same temperature of the mixture.

Cryogenics is the branches of engineering that involve the study of very low temperatures, how to produce them, and how materials behave at those temperatures.

The cryogenic temperature range has been defined as from −150 °C (123 K) to absolute zero (−273 °C), the temperature at which molecular motion comes as close as theoretically possible to ceasing completely.

COP of Heat Pump is defined as the ratio of heat transfer to hot reservoir to net work transfer to the heat pump.

The absorption refrigeration system differs fundamentally from vapor compression system only in the method of compressing the refrigerant. An absorber, generator and pump in the absorption refrigerating system replace the compressor of a vapor compression system.

The two commonly used refrigerant pairs are ammonia – water and lithium bromide – water. 

In Evaporator: As the liquid refrigerant flows through the evaporator, it absorbs heat and changes from the liquid state to a saturated vapour.

In Compressor: Due to the work done on it during compression, the gas further superheated. Therefore the temperature of the discharge gas will be much higher than the saturation temperature of vapour corresponding to discharge pressure.

In Condenser: In the condenser, the temperature of the superheated vapour has to be brought down to its saturation temperature before it can be condensed into a liquid.

The exit condition after evaporation is saturated vapour which is compressed in the compressor and become super-heated and then passed to the condenser.

The coefficient of performance is the ratio of heat extracted in the refrigerator to the work done on the refrigerant.

Work done by compressor, W_in = Q_H - Q_L (Heat of compression) = h₂ - h₁ (Enthalpy increase in compressor)

Vapour compression cycle consists of phase change from gas to liquid in condenser and the compressor pumps liquid phased refrigerant. Making it more efficient as compression work per unit of mass flow is low. It is generally used in refrigeration purposes and air conditioning in buildings.

Air cycle uses air as working fluid and doesn't have phase change in any part of cycle. Meaning air always remains in gas state. Hence the fluid circulation per KW of cooling is very high. Also, the compression work is more. One of very few applications which satisfy its suitability is air conditioning in aircraft.

So (COP)_air < (COP)_vap.c

In the evaporator working below freezing point temperature of water, frosting is a very common phenomenon. Due to frosting the ice is formed on the evaporator tubes. As ice is a bad conductor of heat this will decrease the heat transfer.

Required Properties of Ideal Refrigerant:

1) The refrigerant should have low boiling point and low freezing point.

2) It must have low specific heat and high latent heat. Because high specific heat decreases the refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the refrigerating effect per kg of refrigerant.

3) It must have high critical pressure and temperature to avoid large power requirements.

4) The pressures required to be maintained in the evaporator and condenser should be low enough to reduce the material cost and must be positive to avoid leakage of air into the system.

5) It should have low specific volume to reduce the size of the compressor.

6) It must have high thermal conductivity and high heat transfer coefficient to reduce the area of heat transfer in evaporator and condenser.

7) It should be non-flammable, non-explosive, non-toxic and non-corrosive.

8) It should not have any bad effects on the stored material or food, when any leak develops in the system.

9) It must have high miscibility with lubricating oil and it should not have reacting properly with lubricating oil in the temperature range of the system.

10) It should give high COP in the working temperature range. This is necessary to reduce the running cost of the system.

Units of refrigeration is commonly expressed as ton of refrigeration.

One ton of refrigeration is defined as the capacity to freeze one ton of water from and at 0°C in 24 hours (or refrigerating effect produced by the melting of 1 toon of ice from and at 0°C in 24 hours).

1 TR = 210 kJ/min = 3.5 kW

Throttling devise is another vital part of all the refrigeration systems and air conditioning systems apart from the compressor, condenser and the evaporator.

The throttling devices are also called as the expansion valves because when the refrigerant passes through them the pressure of the refrigerant drops down or it expands.

The refrigerant leaving the condenser is at high pressure. The pressure of the refrigerant has to be reduced so that it can vaporize at the required temperature in the evaporator.

Air Refrigeration System and Bell-Coleman Cycle or Reversed Brayton Cycle:

• In air refrigeration system, air is taken into the compressor from atmosphere and compressed.
• The hot compressed air is cooled in heat exchanger upto the atmospheric temperature (in ideal conditions).
• The cooled air is then expanded in an expander. The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion.
• The low temperature air coming out from the expander enters into the evaporator and absorbs the heat. The cycle is repeated.

R-12 or Freon 12 is used in domestic refrigerators and freezers, liquid chillers, dehumidifiers, ice makers, water coolers, water fountains and transport refrigeration. The wide range of applications of the refrigerant are due to its safe properties.

Properties of R12 and its Advantages

1) Safe properties: Refrigerant R12 is nontoxic, nonflammable, and non-explosive.

2) Suitable for wide range of operating conditions: R12 has the boiling point of -29.8°C due to which it condenses at the moderate pressures at the atmospheric temperature. This means the discharge pressure of the compressor should be only moderate. This helps in using the compressor of low compression ratio that has higher efficiency.

3) Miscibility with oil: Refrigerant R12 is miscible with the compressor oil under all the operating conditions. So, there is no problem of the oil return back to compressor.

R12 has low refrigerating effect.